Heavy-chain antibodies
A heavy-chain antibody is an antibody which consists only of two heavy chains and lacks the two light chains usually found in antibodies.
In common antibodies, the antigen binding region consists of the variable domains of the heavy and light chains (VH and VL). Heavy-chain antibodies can bind antigens despite having only VH domains. This observation has led to the development of a new type of antibody fragments with potential use as drugs, so-called single-domain antibodies.
In 1989 a group of biologists led by Raymond Hamers at the Free University Brussels investigated the immune system of dromedaries. In addition to the expected four-chain antibodies, they identified simpler antibodies consisting only of two heavy chains. This discovery was published in Nature in 1993. In 1995 a research team at the University of Miami found a different type of heavy-chain antibodies in sharks.
The immunoglobulin new antigen receptor (IgNAR) of cartilaginous fishes (for example sharks) is a heavy-chain antibody. IgNAR shows significant structural differences to other antibodies. It has five constant domains (CH) per chain instead of the usual three, several disulfide bonds in unusual positions, and the complementarity determining region 3 (CDR3) forms an extended loop covering the site which binds to a light chain in other antibodies. These differences, in combination with the phylogenetic age of the cartilaginous fishes, have led to the hypothesis that IgNAR could be more closely related to a primordial antigen-binding protein than the mammalian immunoglobulins. To test this hypothesis, it would be necessary to discover IgNAR or similar antibodies in vertebrates that are phylogenetically still older, like the jawless fish lamprey and hagfish.Non-vertebrates do not have antibodies at all.
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